This is a post I did for my blog a few years ago, it may help with the diode question for running two locos from one decoder. I've done it many times now.
-James
James, you are using a diode voltage-dropping circuit which has been around for many decades. Like you mention, it is a handy way to reduce the motor's voltage (each typical silicon diode drops about 0.7V across its internal junction). The diode pairs and the motor are connected in series so, the location of each component doesn't matter. You can have each pair of diodes on each side of the motor, or both pairs can be on the same motor lead. In fact, this circuit was used in DC days to provide constant headlight lighting. Like this:
Diodes_bridge_volt_drop_circuit3.png
Since that double pair of diodes drops around 1.4V, the 1.5V bulb will glow brightly even before the motor starts. And since the voltage drop across the diodes is pretty much constant throughout the loco's operating range, the bulb will glow with steady intensity regardless of the track voltage and loco's speed.
Those diodes can be assembled several different ways and the result will be the same. That presents an opportunity to substitute the 4 discrete diodes with a bridge rectifier, as shown in the diagram below. But to make the bridge rectifier work properly, the DC output leads of the bridge rectifier have to be bridged by a piece of wire (as shown). This works really well in installations where the space is tight (since a bridge rectifier will take up less room than 4 separate power diodes of equivalent amperage rating). Plus, it results in much cleaner and neater circuit.
Diodes_bridge_volt_drop_circuit1.png
Here are some examples of discrete diodes and a bridge rectifier all dropping 1.4V.
Diodes_bridge_volt_drop_circuit2.png
This circuit (from single pair to multiple diode pairs) was used in the DC days to try to somewhat speed-match locos which had greatly different speeds. But I'm wondering why do you need to slow down one of the paired locos when they are identical models? They should be pretty well speed matched to begin with.
